Journal of Molecular Catalysis, 48 (1988)
249 - 264
249
CATALYSIS OF COBALT-SCHIFF BASE COMPLEXES IN OXYGENATION OF ALKENES: ON THE MECHANISM OF KETONIZATION AKIRA NISHINAGA*, TADAO YAMADA, HIROSHI FUJISAWA, KUNIHIKO ISHIZAKI, HIROYASU IHARA and TERUO MATSUURA Department
of Synthetic
Chemistry, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606
(Japan) (Received September 10,1987;
accepted July 15,1988)
A study showing a unique mechanism for a cobalt-Schiff base complexcatalyzed monooxygenation of alkenes is reported. Four-coordinate cobalt(II)-Schiff base chelate complexes catalyze efficiently, in primary or secondary alcohols under atmospheric oxygen pressure at 60 “c, the oxygenation of alkenes substituted with an aromatic or an electron-withdrawing group, which results in ketonization without carbon-carbon bond cleavage. Kinetic studies on the oxygenation of styrene in benzyl alcohol show that the reaction proceeds as a co-oxidation of these substances with a 1: 1 stoichiometry. The proposed mechanism involves rate-determining decomposition of a benzyl alcoholatocobalt(II1) complex to benzaldehyde and a hydridocobalt species, followed by rapid addition of the hydride intermediate to styrene, and dioxygen incorporation into the resulting organocobalt complex.
Introduction Cobalt(II)-Schiff base complexes have received much attention because of their characteristic reactivities in oxygenation reactions. Five-coordinate cobalt(II)-Schiff base complexes have been long known to interact reversibly with molecular oxygen in aprotic solvents to form dioxygen complexes [ 11. Interestingly, when organic compounds, which are related to substrates for some dioxygenase reactions, are put in the reversible oxygenation systems using cobalt(II)-Schiff base complexes, dioxygenase-type reactions
*Present address: Department of Applied Chemistry, Osaka Institute of Technology, Ohmiya 5, Asahi-ku, Osaka 535, Japan. 0304-5102/88/$3.50
@ Elsevier Sequoia/Printed in The Netherlands
250
take place. For example, the oxygenation of 2,6-di-t-butylphenols with Co( L’) gives peroxyquinolatocobalt( III) complexes quantitatively [ 21, and Co(L’ ) catalyzes the oxygenation of 3-substituted indoles and flavonols, resulting in oxidative cleavage of their heterocyclic rings [3]. The most striking characteristic of these model dioxygenase reactions seems to involve a nonradical nature for the dioxygen incorporation step, although the first row transition metals are generally involved in radical chain autoxidation processes. The nonradical dioxygenation process may be understood by assuming a charge transfer (CT) complex between dioxygen and the substrate anion species as the transition state [ 3, 41. The function of the cobalt complexes in these model reactions may be acceleration of the conversion of the CT complex as well as stabilization of the resulting peroxidic product by way of coordination [4]. Alkenes are normally not oxidized under these mild reversible oxygenation conditions. These cobalt(II)-Schiff base complexes are, on the other hand, oxidized irreversibly with molecular oxygen in alcohols to give hydroxo- or alkoxocobalt(II1) complexes [5, 61, which promote monooxygenations of phenols [7] and hydrazones [8]. Recently, Zombeck and coworkers reported that the catalytic oxygenation of 1-alkene with Co(L8) in ethanol at an elevated temperature gave a mixture of 2-alkanone and 2-alkanol [9], and proposed a mechanism involving addition of a hydroperoxocobalt(II1) complex intermediate to the alkene substrate, on the basis of the finding that similar results were obtained with hydrogen peroxide in place of molecular oxygen [lo]. Our recent interesting findings on the reduction of hydroxo- and alkoxocobalt( III)-Schiff base complexes with alcohols to the corresponding cobalt(I1) species [ll] prompted us to investigate the mechanism of the interesting oxygenation of alkenes with cobalt(II)-Schiff base complexes. We find that in the presence of a primary or secondary alcohol, Co(L’) and its related four-coordinate cobalt(II)-Schiff base chelate complexes are effective for the catalytic oxygenation of p-substituted styrenes and related alkenes, whereas five-coordinate complexes, including Co(L’) [9, lo] are not very effective. The predominant reaction observed in the present work is ketonization without degradative cleavage of any carbon-carbon bond in the alkene substrate. In addition to the Wacker process [12 - 151, several methods, including 02/RhC1s/CuC12/EtOH [ 161, Oz/Rhl/MeOH [ 171, 02/IrHC13(CsHi2)/HZ [18], H202/Pdn [19], 0,/Co(TPP)(N0,)(Py)/Pd(OAc)2 [20], Pd(MeCN)2Cl(N02) [21] and t-BuOOH/Pd(OCOR)2 [22], have been reported for the ketonization of alkenes. Mechanisms for most of them are considered to involve activation of oxygen species such as Os, HzOz or Hz0 by the metal complexes. Contrary to the mechanism proposed by Hamilton et al. [lo], it is concluded that the key step of the present oxygenation involves addition of a hydridocobalt complex, generated by reaction between the catalyst and the alcohol solvent, to the alkene substrate, followed by dioxygen insertion into the resulting organocobalt complex intermediate.
251
Results Oxygenation of alkenes in alcohols When Co”(L’) is used as a catalyst in methanol or ethanol at 60 “C, the oxygenations of substituted ethylenes 1,indene 5 and 1,2-dihydronaphthalene 6 take place smoothly under atmospheric pressure of oxygen. Main products are ketones 2, 7 and 8 (Table 1). As seen from Table 1 (Runs 4 and 14), Co"(L' ) is much more effective than Co”(Ls) employed in the literature [lo], indicating that a four-coordinate structure is important for the catalytic activity of the cobalt(I1) complex. Since cobalt(R)-Schiff base complexes are rapidly oxidized with O2 in methanol or ethanol to give the corresponding hydroxocobalt(II1) complexes [6], the initial catalytically active species should be Co”‘(L)(OH). Actually, the same results are obtained with Co”‘(L’)(OH) in place of Co”(L’) (Table 1, Runs 4 and 13). The rate of the oxidation depends on the reaction temperature: the maximal rate is obtained at 60 “C, whereas at room temperature almost no reaction takes place (Table 1,
TABLE
1
Cobalt-Schiff
base complexcatalyzed
oxygenation
of alkeneP
Run
Alkene
Co(L)
Solvent
Reaction temp. (“C)
Reaction time (h)
Conversion Product yield (%)b (%) 2 3 Other
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
la la la la la la la la la la la la la la lb lc Id le If lg 5 6
Co(L’) WL’ ) Co(L’) Co(L’) Co(L’) Co(L’) Co(L’) Co(L’) Co(L’) Co(L’)(OH) Co( L3) Co(L’) Co(L’)(OH) Co(LB) Co(L’ ) Co(L’) WL’) co&j Co(L’)d Co(L’) Co(L’) Co(L’)
MeOH MeOH MeOH MeOH MeOH MeOH EtOH i-PrOHC t-BuOHc MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH EtOH EtOH
25 40 50 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 50 60 60 60
23 23 24 22 10 4 12 8 48 20 24 24 20 24 22 14 12 10 24 70 24 40
0.1 10 19 100 40 14 100 100 0 100 20 11 12 32 100 100 100 100 91 48 40 60
*Alkene (2 mmol), Co(L) (0.2 mmol), solvent (10 ml), 02 (1 atm). bDetermined by GLC using tetradecane as an internal standard. ‘CHzClCHzCl(20 ml) was added to dissolve the cobalt complex. dAn equivalent of Co(L’) was employed (no catalytic reaction).
-
91 97 91 91 91 83 85
91 89 61 60 53 81 89 17 10 100 34 90 (7) 65 (8)
3 3 9 9 9 17 15 9 11 33 40 47 13 11 70 60 -
10 (4) Co(L’)(CN)
252
Runs 1 - 4), and on the nature of the alcohol used: the rate increased in the order; MeOH < EtOH < i-PrOH (Table 1, Runs 4, 7, 8). No reaction takes place in t-butyl alcohol (Table 1, Run 9) or in aprotic solvents. These results are compatible with those for the reduction of Co”‘(L’)(OH) with alcohols [ 111. In the oxygenation of p-substituted styrenes, compounds 2 and 3 are the sole products. The product ratio 2/3 is found to be constant throughout the reaction course, although the ratio depends on the nature of the alcohol as well as the cobalt complex: a larger amount of 3 is obtained when ethanol or isopropanol is used as the solvent and Co”(L’) or Con(L8) as the catalyst (Table 1, Runs 7,8,12 - 14). R’R2C=CH2 + 0 2
R’COCHs + R’R’C(OH)CHs
=
1 a; c; e; g;
2
R’ = Ph, R2 = H R’ = 4-Cl&H‘+, R2 = H R’ = R2 = Ph R’ = COOEt, R2 = H
Ph,C(OCHs)CH,
Co(L3)
; z= a
CO(L’) : Z = (CH*)4
b; d; f;
R’ = 4-MeOCgH4, R2 = H R’ = Ph, R2 = Me R’ = Me, R2 = CN
’ 1 ’ 60
4
Co(L’) ; Z = (CH&
3
5
~~(~2)
6
7
8
; z = a
CO(L’) : Z = (CH*)3 Co(LS)
Co(L7) : Z = (CH2)3NHKH2)3
: z = -, w
-
Me Ma
Co(L*) : Z = (CH2)3NMe(CH2)3 Co(L’)
; Z = 2 n-Pr
Since no oxidation of 3a to 2a takes place under the reaction conditions, these products should result from a common intermediate, PhCH(Me)OOH. Actually, the decomposition of PhCH(Me)OOH in methanol in the presence of Co”‘(L’)(OH) at 60 “C gives 2a (92%) and 3a (8%). No significant effect of the p-substitutent on the reactivity of p-substituted styrenes has been observed (Table 1, Runs 4,15,16). Similarly, o-substituted styrenes Id and le are readily oxygenated to 2 and 3 (Table 1, Runs 15,16). In the case of le, however, the formation of 4 is remarkable. The cobalt catalyst is essential for the formation of 4 from le, and 4 is not formed from 3a under the oxygenation conditions. Interestingly, however, when a solution of le and Co”‘(L’)(OH) (0.3 equivalent) in
253
methanol is warmed at 60 “C under nitrogen for 3 h, only 4 is obtained in 21% yield (based on Co”‘(L’)(OH) employed) from le,and Co”‘(L’)(OH) is reduced to Co”(L* ). The oxygenation of If proceeds only stoichiomet&ally, where acetone and Co”*(L’)(CN) are formed. An acetone cyanohydrin complex, (CH3)2(CN)COCo111(L’), may be assumed as the immediate intermediate. Co”‘(L’)(CN) is not active for the present oxygenation. Compound lg undergoes the catalytic oxygenation, on the other hand, to give ethyl pyruvate 2g. Interestingly, compounds 5 and 6 are oxidized to ketones 7 and 8, respectively, although @-substituted styrenes such as trans-PhCH=CHX (X = Me, Ph, Br, COOMe), cis-stilbene, and coumarin are not oxidized under similar reaction conditions. Simple aliphatic olefins such as 1-hexene, 2hexene and cyclohexene are less susceptible to the oxygenation (no reaction takes place under conditions similar to those shown in Table 1 in 30 h). Stoichiometry The use of benzyl alcohol allows convenient determination of the stoichiometry and kinetics of the present oxygenation. Figure 1 shows the time course of the Co”‘(L’)(OH) catalyzed oxygenation of la with an excess of PhCHzOH in CH,CICHzC1 at 60 “C under atmospheric pressure of oxygen, and Table 2 summarizes some characteristics of the reaction. As seen from these results, the reaction proceeds moderately (89% conversion of la in 31 h) to give 2a and 3a with high selectivity (98%) and a constant product ratio (2a/3a = 4.2) throughout the reaction course. It is therefore clear that PhCHO results only from PhCHzOH and not from la. Since the oxidation of PhCHzOH in the absence of la proceeds only at a rate of 4.5 X low3 mmol h-l (Table 3, Run 7) and no oxidation of la takes place without the alcohol,
time
(h)
Fig. 1. Time course of the oxygenation of styrene (0.436 mmol) with benzyl alcohol (483.0 mmol) catalyzed by Co(L’)(OH) (0.05 mmol) in CHzClCHzCl(l0 ml) under oxygen atmosphere (760 mmHg) at 60.0 f 0.1 “C. la (O), 2a (a), 3a (A), PhCHO (a), PhCHO in the absence of la (a).
254 TABLE 2 Characteristics Fig. 1
in Co(L’ )(OH) catalyzed oxygenation
Time (h) Selectivitya Co-oxidn. ratiob Product ratioC
2
4
6
8
9
98.5 1.5 4.3
98.3 1.5 3.9
99.6 1.4 4.1
97.4 1.4 4.2
98.3 1.4 4.1
*(la + 2a + 3a)/(la)ifi$id bPhCHO/( 2a + 3a). O2a/3a.
of la under conditions shown in
23
27
31
96.5 1.3 4.3
95.2 1.3 4.3
97.6 1.3 4.3
97.7(average) 1.4(average) 4.2 (average)
X 100.
it is noted that the oxidation of PhCHzOH is accelerated by la and that the oxidation of la occurs at the expense of an equimolar amount of PhCHzOH: that is, the reaction proceeds as a co-oxidation of la and PhCHzOH with 1:l stoichiometry (the co-oxidation ratio, PhCH0/(2a + 3a) = 1.3, is constant throughout the reaction course).
Kinetics In the Co”‘(L’)(OH)-catalyzed oxygenation of la with PhCH,OH, the reaction rate depends on the concentration of PhCHzOH. Both of the cooxidation and product ratios also depend on the amount of PhCHzOH employed: PhCHO/(Za + 3a) increases and 2a/3a decreases on increasing the amount of PhCHzOH (Table 3, Runs 1 - 6). The results suggest that 3a is formed at the expense of PhCH20H. An assumption of hydrogen abstraction by the PhCH(Me)O’ radical, the common intermediate for 2s and 3a, from PhCHzOH is reasonable. The reaction rate is independent of both the concentration of la and the oxygen pressure, where both the co-oxidation and product ratios are not much changed (Table 3, Runs 3, 9, 11 - 15). The reaction rate depends also on the concentration of Co"'(L' )(OH), although no quantitative relation can be obtained because the catalyst decomposes with time, and the deactivation is not linearly correlated to the concentration of the catalyst. Effects of additives on reaction rate The addition of N-methylimidazole (NMeIm) (iVMeIm/Conl(L’)(OH) = 2) inhibits the oxidation of la nearly completely. On the contrary, when PPh, is added to the oxygenation system (PPhJCo = lo), the reaction rate increases remarkably with a decrease in the product ratio 2a/3a. Interestingly, however, further addition of PPhs (PPhJCo = 50) decreases the reaction rate (Table 3, Runs 18, 19). Furthermore, in the Co”‘(L’)(OH)-catalyzed oxygenation of la in methanol (60 “C, 1 atm O,), the addition of t-BuOK serves to slow down the reaction rate, with a decrease in the product ratio (see Experimental Section).
45.8 45.8 45.8 45.8 45.8 45.8 0.0 274.8 423.7 49.5 44.6 46.1 46.5 46.1 46.3 47.6 47.5 42.8
42.5
44.2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
19
20
483.0
483.0
9.66 48.3 96.6 193.2 483.0 966.0 96.6 98.4 101.8 483.0 483.0 483.0 483.0 483.0 483.0 483.0 483.0 483.0
PhCHaOH (mM)
O2 (atm)
Reac. temp. (“C)
P;h;,
250 6moh)
5.0 1.0 40 (with PPhs, 250 mM)
;:th
5.0 1.0 60 5.0 1.0 60 5.0 1.0 60 5.0 1.0 60 5.0 1.0 60 5.0 1.0 60 5.0 1.0 60 5.0 1.0 60 5.0 1.0 60 2.5 1.0 60 5.0 1.0 60 5.0 1.2 60 5.0 0.8 60 5.0 0.6 60 5.0 0.2 60 5.0 1.0 40 5.0 1.0 50 5.0 1.0 60 (with PPha, 50 mM)
‘Co’ (mM)
15
5
40 29 30 30 31 30 31 30 30 30 28 24 27 27 30 29 29 8
Reac. time (h)
99.3
58
48 25 73 77 74 79 16 44 88
5.2 40 60 81 89 89 -
Conv. (%)
78.0
39.4
7.58 21.6 33.1 46.2 50.0 55.9 12.9 32.9 35.3 32.5 14.6 39.9 47.9 44.8 44.2 12.7 27.1 49.3
15.4
11.8
20.9 22.5 17.7 7.9 23.4 28.1 26.5 28.6 6.4 15.3 23.2
15.6 23.2 30.2 30.6 28.8 -
6.94
PhCHO 2a (mM) (mM)
oxygenation of la with PhCHaOH in CHaClCHaW
25.9
14.3
2.5 2.9 5.3 2.4 6.2 7.4 7.0 5.9 1.7 4.0 9.5
0.41 1.7 3.3 5.9 7.2 10.9 -
&vi)
95
99
96 94 98 98 92 99 99 97 99 97 98
99 94 98 99 98 98 -
Select.b (%)
1.89
1.51
1.41 1.39 1.41 1.42 1.35 1.35 1.34 1.28 1.57 1.40 1.51
1.03 1.23 1.25 1.28 1.32 1.41 -
Co-oxdn. ratio
0.59
0.8
7.7 6.8 3.4 3.3 3.7 3.6 3.8 4.8 3.7 3.8 2.4
13.8 8.2 6.6 5.2 4.2 2.5 -
2a/3a
*Data were obtained by GLC analysis using tetradecane as an internal standard. Accuracy of reaction temperature = kO.1 “C. bSelectivity: see footnote Table 2. CObserved fist order rate constant determined by the time-dependent consumption of la.
la (mM)
Run
Co(L’)(OH)-catalyzed
TABLE 3
2.72
4.63
1.54 1.50 1.48 1.39 0.16 0.53 6.48
0.17 0.56 0.93 1.47 1.67 2.12 -
(s-’ )
kc x 10’
256
Effects of stmcture of Co(L) on reaction rate
The catalytic oxygenation of la is notably influenced by the structure of the cobalt catalyst. Five-coordinate cobalt-Schiff base complexes such as Co”‘(L’)(X) (X = Cl, OAc, CN), Con(L7), Co”(L’), Con(L1)(IVMeIM), fourcoordinate complexes such as Con(L3) and Co”(TPP) in which the planar chelate is not flexible, and non-chelate complex, Co(Lg), are all less reactive. It is noted that nonplanar complexes, Co(L4)-Co(L6), are highly reactive compared to the planou ones, Co( L’ )-Co(L3), and that the product ratio 2a/3a decreases remarkably with these nonplanar complexes (Table 4). Oxygenation of la with PhCDzOH The Con'(L' )(OH)-catalyzed
oxygenation of la using PhCD*OH in place of PhCHzOH gives a reaction mixture composed of mono- and nondeuterated acetophenones and l-phenylethanols, obtained with the product ratio acetophenones/l-phenylethanols = 4.7 (Fig. 2). The results clearly PhCH=CH2 + PhCDzOH + 0s -
PhyCH, + PhyCH*D + PhCHCH, b
hH
b
+ PhCHCH,D dH
1
fb1
_i,_ 2.3
27
26
25
21
23
’
Fig. 2. 13C NMR, wide “H decoupled (lower) and off-resonance ‘H decoupled (upper), of a mixture of (a) PhCOCH3 and PhCOCH2D, and (b) PhCH(OH~H~ and PhCH( OH)CHsD obtained from the oxygenation of styrene with PhCDzOH; only the methyl group region of each mixture is shown.
257
258
indicate that the methyl group of the product arises from hydrogen transfer from the o-position of benzyl alcohol to the terminal methylene carbon of la, which is mediated by the catalyst. The formation of the mixture comprised of mono- and nondeuterated products may be rationalized by assuming the reaction of PhCH=CH, with Co(L’)(H) and Co(L’)(D) formed by the following reactions [23] : Co(L’)(OCD2Ph) Co(L’)(D)
-
Co( L’
+ PhCD20H =
)(D)+ PhCDO Co(L’)(H)
+ PhCDzOD
The use of PhCD,OD gives only PhCOCHzD and PhCH(OH)CH2D. Oxygenation of la with Co(L’)(OCH2CC13) The oxygenation of la with Co(L’)(OCH2CC1s) (1 mmol) in CH2CICHZC1 at 60 “c gives PhCOMe (0.47 mmol) and PhCHO (0.28 mmol), although no reaction takes place at room temperature. The results indicate that no alcoholic solvent is needed for the oxygenation of styrene and provide evidence that the alcoholatocobalt(II1) complex can function as a hydrogen carrier at a higher temperature. The results also suggest that 1-phenylethoxy radical is formed as the intermediate that undergoes elimination of either o-hydrogen or a methyl radical in the absence of a hydrogendonating alcohol. Oxygenation of 4-phenyl-1 -butene In order to examine the structural requirement of the alkene substrate for the present oxygenation, the catalytic oxygenation of 4-phenyl-1-butene, a nonconjugated olefin, with Co”‘(L’)(OH) (0.1 equiv) has been carried out in CH2C1CH2C1 containing PhCHzOH (11.1 equiv) at 60 “C under 1 atm oxygen. Although the reaction is very slow (turnover: 2.7 in 90 h), PhCHO (co-oxidation ratio = 4.73), 4-phenyl-2-butanone and 4-phenyl-2-butanol (product ratio = 2.25) are obtained with high selectivity (98.6%). The reaction is first-order in PhCH?OH (kobs = 9.58 X 10e7 s-i) and the rate of formation of PhCHO is 4.78 X 10m3 mmol h-l, nearly equal to that observed in the oxygenation of the alcohol alone (de supra). Discussion Mechanism for oxygenation of la with PhCH,OH The results obtained in stoichiometric and kinetic studies on the oxygenation of la with PhCHzOH described above are rationalized by the reaction diagram in Scheme 1. The incorporation of the a-hydrogen of PhCHzOH into the double bond in la, demonstrated by the oxygenation with PhCDzOH, is understood by assuming the addition of hydridocobalt complex 10 to la to form phenethylcobalt complex 11.The peroxymetallation mechanism proposed for the ketonization of l-alkenes with molecular oxygen catalyzed
259
PhCH20H
Products
PhCH,OH
c~‘oH)---&com~H2~
Phil$OH Co=OOCHCHa
CoH
t3
-
PI&
l
14 ConOCHCH3 +h
Ph
Scheme 1.
11
by RhCls-CuC12 in ethanol [16] may not be applicable, because there is no way by this mechanism to explain the specific hydrogen transfer from the alcohol to the alkene double bond. The hydridocobalt complex 10 is considered reasonably to result from thermal decomposition of the alcoholato complex 9, produced initially by an acid-base reaction between Co”‘( L1 )( OH) and PhCHzOH as proposed for the reduction of Co”‘(L’)(OH) with alcohols [ 111. The kinetic results can be explained in terms of the rate-determining decomposition of 9 to 10, which then reacts rapidly with la to form 11. The effect of the concentration of PhCHzOH on the reaction rate is attributed to the concentration of 9. It is noted that despite the high reactivity of hydridocobalt complexes toward molecular oxygen [24], complex 10 reacts faster with la than with O2 under the present reaction conditions. This may be due to a low concentration of dioxygen in the solution at such high temperature. The reaction sequence, 9 + 10 3 11, is also consistent with the formation of 4 from le, where the nucleophilic attack by methanol on the carbon of the Co-C bond in MePhzCCo(L1) intermediately formed may be possible. Although there is no example of the nucleophilic substitution on the carbon in organocobalt complexes, heterolysis of the Co-C bond giving Co’ and a carbonium ion may be possible [25]. The phenethylcobalt complexes of type 11 are known to be susceptible to dioxygenation, giving rise to peroxocobalt(II1) complexes of type 12 [26, 271. Since the rate of the present oxygenation is independent of oxygen pressure, the dioxygenation step is a fast process. The resulting peroxo complex 12 then reacts with PhCHzOH to give hydroperoxide 13 and 9, completing the catalytic cycle. The interesting effect of PPh3 on the .acceleration of the reaction as well as on the decrease in the product ratio 2a/3a, is explained by the increase in the concentration of 9 resulting from the reduction of peroxo complex 12 with PPhs to give Ph(Me)CHOCo(L’) 15 [28]. Actually, Ph,PO is isolated in this case. Since the alkoxy anion in 15 is a stronger base than the alkylperoxy anion in 12, the formation of 9 by reaction of PhCHzOH with 15 is naturally much more effective than that with 12. The reverse effect of a larger excess of PPh3 (PPhs/Co = 50) on the reaction rate may be due to inhibition of the generation of 10 caused by the coordination of the
260
phosphine to the cobalt center. The observed retardation of the reaction rate by the addition of t-B&K also supports involvement of the addition of the hydridocobalt complex to styrene, because the addition reaction of the resulting anionic Co* species to styrene is considered to be slower than that of the hydridocobalt complex [ 291, The slow reaction and the high co-oxidation ratio observed for the Co(L’)(OH)-catalyzed oxygenation of PhCH,CH2CH=CH2 with PhCHzOH indicate that the oxidation of PhCHzOH occurs in the main catalytic cycle, to which the oxygenation of the alkene is linked as shown in Scheme 2. One cycle of the oxygenation of PhCH~CH~CH=CH~ occurs during -5 cycles of the alcohol oxidation taking place. The reactivity of alkenes towards the present oxygenation corresponds to that in the addition reaction with hydridocobalt species [29a]. Since the formation of 10 from 9 is the rate-determining step in the oxidation of benzyl alcohol with Co(L’)(OH) [ll], the reaction rate, as well as the efficiency of the alkene oxygenation, depends on the rate ratio u,,/u,~ in Scheme 2.
Products
Scheme 2.
These findings, particularly the effects of the additives on the reaction rate and the results obtained in the oxygenation of la with Co(L’)(OCH2Ccl,) cannot be explained by the mechanism proposed by Hamilton et al.
[W. Structural requirements of the cobalt complex Activation process The reaction path, 9 + 10, in Scheme 1 proceeds as the monomolecular decomposition of 9, as demonstrated in the reduction of Co(L' )(OH) with alcohols [ll]. The interesting finding that the oxygenation of styrene using Co(L4), Co(L’) or Co(L6), in which the Schiff base Iigand is not planar [30, 311, is faster than that using planar Co(L') (Table 4) suggests that the transition state with a twisted conformation 16 is important for the formation of 10 (Scheme 3). In order to attain the twisted conformation 16, the flexibility of the Schiff base ligand as well as a coord~atively unsaturated structure of the complex is important. This is supported by the fact that Co(L3), Co(TPP), Co(L’) and Co(L*) are all less effective. There seems to be no correlation between the reactivity and the redox potential of these complexes [ 32 J. The extreme retardation of the oxygenation of la by the addition of MvIeIM is due to coordination of the base, resulting in inhibition of the conformation change [ll]. The faster reaction with Co(L2) than with Co(L’) indicates
261
Cd’i’,CHzR~
0’
,6
CH2R
Scheme 3.
that the flexibility of the ligand is much more important than the steric factor, because the planarity of the ligand in Co(L’ ) is better than that in Co( L2), whereas L2 is bulkier than L' . It is therefore concluded that the use of coordinatively unsaturated Co”‘-Schiff base chelate complexes is essential for effective catalysis in the present oxygenation. The ineffectiveness of non-chelate complex Co(L’) may be due to its lack of susceptibility to molecular oxygen under the reaction conditions because of its high oxidation potential. All other Schiff base complexes, Co(L* ) - Co(L’), are readily oxidized to Co”’ species with molecular oxygen in the presence of an alcohol. The nonreactivity of Co(L’ )(X) (X = Cl, OAc) is due to the strong acidity of the conjugate acid of the ligand X, which is eventually unfavorable to the formation of 9. Product distribution As stated above, there is no doubt that in the oxygenation of la the immediate precursor of the products is the 1-phenylethoxy radical 14, which gives 3 when an excess of alcohol is present. The larger formation of 3 in i-PrOH and in EtOH than in MeOH (Table 1) is indicative of the o-hydrogen abstraction by 14 from the alcohols. The question arises why the product ratio 2a/3a decreases, that is, the formation of 3a increases, when the catalysts Co(L4) - Co(L6) are used (Table 4). In general, four-coordinate cobalt chelates, such as Schiff base and porphyrin complexes, can interact significantly with molecular oxygen in nonpolar aprotic solvents only when an axial nitrogen base ligand is present [l]. Actually, Co(L1) is not susceptible to molecular oxygen in CH2C12 or CH2ClCHzC1 without an axial base ligand, whereas in primary and secondary alcohols it is oxidized with molecular oxygen, even in the absence of such an axial nitrogen base, to give diamagnetic Co(L' )(OR) [6]. Interestingly, however, we find that nonplanar four-coordinate complexes, Co(L4) - Co(L6) are quite susceptible to molecular oxygen, even in CH2C12 or CH2ClCH2C1without any axial base ligand, to give paramagnetic cobalt(II1) species [32]. Therefore, the interesting decrease in the product ratio 2a/3a is probably due to prolongation of the lifetime of ‘14 by a paramagnetic, associative inter-
262
action with these paramagnetic cobalt(II1) species [33]. A similar interaction may be considered for the reaction with Co(L’). The increase of 2a in the cases with Co(L’) and Co(Ls) may be correlated to the results in the reduction of peroxy-pquinolatocobalt( III)( L’) complexes with alcohols [ 341. The cobalt catalysts are normally inactivated after about 10 turnover cycles. The mechanism of the inactivation is currently being investigated. Experimental Methods ‘H NMR spectra were determined with a Varian T-60 spectrometer. 13C NMR spectra were recorded on a JEOL JNM-GX400 FT NMR Spectrometer. GLC analyses were performed with a Shimazu Gas Chromatograph GC-7A, which was connected to a Shimazu CRlA Chromatopac recording system. Ma teriuls Alkenes are all commercially available and were purified before use. Cobalt complexes were synthesized according to the known methods: Co(L’) [2], Co(L2) [35], Co(L3) [36], Co(L4) [30], Co(L’) [30], Co(L6) [31], Co(L7) [2], Co(L8) [2], Co(L9) [37]. Oxygenation of alkenes in alcohol: geneml procedure A solution of the alkene substrate (2 mmol) and Co(L) catalyst (0.2 mmol) in an appropriate solvent (10 ml) was warmed at the temperature given in Table 1 under 1 atm oxygen. The conversion of the substrate and the yield of the products were determined by GLC analysis using a column (3 mm X 2 m) packed with PEG (10%) supported on Chromosorb W. The structure of the products was determined by comparison of their ‘H NMR spectra with authentic samples after isolation by TLC of each reaction mixture. The results are given inTable 1. In the case of If, Co(L’)(CN) separated out as fine crystals nearly quantitatively during the course of the oxygenation. Anal. Calcd for C17H14N302C~ (1.5 H,O): C, 53.98; H, 4.53; N, 11.11%. Found: C, 53.88; H, 4.02; N, 10.91%. Kinetics: typical procedure To a solution of la (45.8 mM), PhCH20H (0.483 M) and Co(L’)(OH) (5.0 mM) in CH2C1CH2C1 (10 ml) was added tetradecane (25 ~1) as an internal standard. In the oxygenation of 4-phenyl-1-butene, dodecane was employed as an internal standard. The reaction vessel was then filled with 1 atm oxygen and put in a thermoregulated water bath (60.0 f 0.1 “C). Aliquots (0.3 ml) were taken out at the intervals. shown in Fig. 1, put on a short silica gel column (5 mm X 10 mm), and eluted with ethyl acetate (0.3 ml X 4) to remove the metal catalyst. The eluent was analyzed by GLC (uide supm). The results are shown in Fig. 1 and Table 2. The data given in Tables 3 and 4 were similarly determined.
263
Oxygenation of la in the presence of t-BuOK A solution of la (45.8 mM), Co(L’)(OH) (5 mM) and t-BuOK (0.1 M) in methanol (10 ml) was warmed at 60.0 + 0.1 “C under 1 atm oxygen. The reaction was monitored by GLC as described above. The initial rate constant was determined to be 2.4 X 10e6 s-r (4.0 X 10e6 s-* in the absence of tBuOK), and the product ratio 2a/3a was 2.0. No retardation of the oxidation was observed when a 10 mM solution of t-BuOK was used. Oxygenation of la with PhCDzOH A solution of PhCDzOH (0.55 ml), synthesized readily by the reduction of PhCOOMe with LiAlD4, la (0.5 ml) in CH2ClCHzC1 (50 ml) containing Co(L’)(OH) (0.17 g, 0.5 mmol) was warmed at 40 “C under 1 atm oxygen for 90 h. The mixture was filtered through a short silica gel column and eluted with ethyl acetate to remove the metal complex. The eluent was evaporated in uacuo and the residue was chromatographed on a silica gel layer to isolate PhCOCH2D and PhCH(OH)CH2D. The 13C NMR of these deuterated products are shown in Fig. 8. Synthesis of Co(L’)(OCH2CC13) To a suspension of Co(L’) (5.44 g, 16.7 mmol) in dichloromethane (50 ml) was added CC1sCH20H (25 g, 0.167 mol). The mixture was then stirred under 1 atm oxygen at room temperature for 3 h. After filtration of undissolved materials, the resulting brown solution was evaporated in uacuo. Co(L’)(OCH,CCls)(CCI,CH,OH) was obtained as fine crystals (8.76 g, 84.2% yield). Anal. Calcd for C20H&16N204CO: C, 38.56; H, 3.07; Cl, 34.14; N, 4.50%. Found: C, 38.71; H, 3.03; Cl, 34.06; N, 4.62%. Oxygenation of la with Co(L’)(OCH2CC13)(CC13CH20H) A solutionof la (50 ~1) and Co(L’)(OCH,CCl,)(CCl,CH,OH) (1 mmol) in CH,ClCH,C1(15 ml) containing tetradecane (25 ~1) as an internal standard was warmed at 60 “C under 1 atm oxygen for 43 h. The GLC analysis indicated the formation of acetophenone 2a (0.47 mmol) and benzaldehyde (0.28 mmol), together with other two products which were not determined. References 1 (a) R. D. Jones, D. A. Summerville and F. Basolo, Chem. Rev., 79 (1979) 139; (b) E. C. Niederhoffer, J. H. Timmons and A. E. Martell, Chem. Rev., 84 (1984) 137. 2 A. Nishinaga, H. Tomita, K. Nishizawa, T. Matsuura, S. Ooi and K. Hirotsu, J. Chem. Sot., Dalton Trans., (1981) 1504. 3 A. Nishinaga and H. Tomita, J. Mol. Catal., 7 (1980) 179. 4 (a) A. Nishinaga, Protein, Nucleic Acid, Enzyme, Vol. 26, Kyoritsu Shuppan Co., Tokyo, 1983, p. 214; (b) A. Nishinaga, H. Iwasaki, T. Shimizu, Y. Toyota and T. Matsuura, J. Org. Chem., 51 (1986) 2257. 5 G. Costa, G. Mestroni and G. Pellizer, J. Organometall. Chem., 15 (1965) 187. 6 A. Niihinaga, T. Kondo and T. Matsuura, Chem. Lett., (1985) 905. 7 A. Nishinaga, H. Tomita and T. Matsuura, Tetrahedron Lett., 21 (1980) 1261. 8 A. Nishinaga, S. Yamazaki and T. Matsuura, Tetrahedron Lett., 27 (1986) 2649.
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